Conventionally, an optical film thickness measurement using an ellipsometer has been employed, for example, in inspecting a film thickness in a semiconductor device manufacturing process.
FIG. 2 schematically illustrates such an optical film thickness measurement using the ellipsometer. As shown in the drawing, in the optical film thickness measurement using the ellipsometer, light emanating from a light source 101 passes through respective optical elements, i.e., a polarizer 102 for regulating a polarization state and a compensation plate 103 for regulating a phase, whereby elliptically polarized incident light is formed and illuminated on a sample 107.
Then, by installing an analyzer 104 for examining the polarization state, a spectroscope 105 for selecting light of a predetermined wavelength, and a detector 106 along an optical path of the light reflected from the sample 107, the polarization state is measured for each wavelength of the reflected light and a spectrum is obtained.
For example, as shown in FIG. 3 in case of measuring a thickness of an oxide film 201 (e.g., a gate oxide film) formed on a Si substrate 202, an elliptically polarized incident light illuminated as described above at an angle θ0 from the air 200 (refractive index=N0) is reflected from a surface of the oxide film 201 (refractive index=N1) and, at the same time, most of the incident light is transmitted into the oxide film 201. Further, the light transmitted into the oxide film 201 is reflected off an interface (a substrate surface) of the Si substrate 202 (refractive index=N2) and returns to the air 200 from the inside of the oxide film 201 to thereby interfere with the light reflected from the surface of the oxide film 201 such that the polarization of the light returned to the air by being reflected from the interface of the Si substrate 202 interferes with that of the light reflected from the surface of the oxide film 201.
As to the aforementioned light, after calculating p-polarization component and s-polarization component, a film thickness is calculated from a phase difference Δ and an amplitude ratio ψ thereof.
P-polarization component r1p and s-polarization component r1s of the light reflected from the surface of the Si substrate 202 are respectively calculated by the following equations.r1p=(n2cos θ1−n1cos θ2)/(n2cos θ1+n1 cos θ2)r1s=(n1cos θ1−n2cos θ2)/(n1cos θ1+n2cos θ2)
Further, polarization states of p-polarization component Rp and s-polarization component Rs of detected light are obtained by the following equations based on the polarization components r1p and r1s described above and p-polarization component r0p and s-polarization component r0s of the light reflected from the surface of the Si substrate 201.Rp=(r0p+r1pexp(−2iδ))/(1+r0p·r1pexp(−2iδ))Rs=(r0s+r1sexp(−2iδ))/(1+r0s·r1sexp(−2iδ)),wherein δ=2π n1 t cos θ1/λ.
Consequently,Rp/Rs=tan(ψ)·exp(−iΔ)
which is used to calculate cos Δ and tan ψ for each wavelength, thereby obtaining a wavelength dependent spectrum. Thereafter, by using a film thickness value t of the oxide film 201 as a parameter, the measured spectrum and the theoretical spectrum are compared to thereby calculate the film thickness value t.
Additionally, when the oxide film 201 formed on the Si substrate 202 has a grating structure as shown in FIG. 4, a cross sectional shape of the grating structure can be also measured by using the ellipsometer and, further, as shown in FIG. 5, there is a method for finding out the shape by dividing a pattern portion into L equal parts and regarding the grating portion as L layer laminated films. In such method, the cross sectional shape is obtained by a model, wherein there is a mixed layer of dielectric constants of air and the grating structure and nth-order diffracted light is generated at each interface. In this case, as shown in FIG. 5, there is a region where the grating pattern is assumed to repeat itself infinitely. Dividing the grating structure into L layers, the dielectric constant is calculated based on a volumetric ratio of air to the grating portion in each layer. By making a model, wherein when light is projected onto the L layer laminated structure at a predetermined angle, respective nth order diffracted lights of reflection and transmission are generated from each interfacing surface of layers, each amplitude reflectivity of s-polarization component and p-polarization component is calculated. Finally, the cross sectional shape can be obtained from wavelength dependency of cos ψ and tan ψ.
Both of the aforesaid film thickness measuring method and cross sectional shape measuring method are applicable to a case where a substrate surface (a reflective surface of a lower layer) is formed of material having a large absorption coefficient k for light (less light-transmissive material) such as Si, Al and Cu and a film is thick, even and non-transmissive.
However, in the aforementioned film thickness measuring method and cross sectional shape measuring method, there is a drawback that a film thickness and a cross sectional shape cannot be measured in case of a sample having a multilayer interconnection structure in a lower layer of an oxide film as a measurement target.
That is, for example, a sample shown in FIGS. 6A to 6C includes a layer formed of an oxide film 301 as a target of film thickness measurement; b layer formed of metal interconnection 302 and an interlayer insulating film 303 therebelow; and c layer formed of metal interconnection 304 and an interlayer insulating film 305 therebelow, as illustrated in FIG. 6A, and has a multilayer interconnection structure with the metal interconnection 302 of the b layer formed to be orthogonal to the metal interconnection 304 of the c layer. In such multilayer interconnection structure, the oxide film 301 and the interlayer insulating films 303 and 305 are formed of highly light-transmissive material such as silicon dioxide and the metal interconnections 302 and 304 are formed of less light-transmissive material such as Cu, Al and W. Further, reference numerals 306 and 307 of FIG. 6A are stopper layers formed of a SiN film or a SiC film and used in a CMP(Chemical Mechanical Polishing) process.
In addition, in case of a sample shown in FIG. 7 the oxide film 301 has a grating structure.
In the sample having the aforementioned multilayer interconnection structure, although a theoretical spectrum is calculated by assuming that light is reflected from the surface of the a layer and the interface between the a layer and the b layer or that light is reflected from the surface of the a layer and the interface between the b layer and the c layer, the film thickness and the cross sectional shape cannot be measured due to a complicated structure of the reflective surface of the lower layer.
Accordingly, conventionally, when measuring a thickness of a film formed on a multilayer interconnection structure, after a test piece wafer is loaded during a manufacturing process in addition to a product wafer and a film as a measurement target is formed on the test piece wafer (bear Si), a film thickness is measured by an optical method.
In the same manner, in measuring an etching shape, after loading the test piece wafer and performing a process such as film forming, resist coating, exposure and etching, a cross section is observed by SEM, thereby controlling a shape thereof.
For this reason, in any case, there is a drawback that the manufacturing cost becomes increased, as the number of test piece wafers is increased.
Furthermore, after forming a monitoring pattern for measuring the film thickness or the etching shape in a chip, the position thereof can be measured by using a conventional method. However, in the trend of high integration of IC, it is difficult to provide a monitoring pattern of a predetermined size in the chip. Moreover, granted that it is provided, there is a drawback that dishing, which means that the surface of the metal interconnection 302 is abraded into a bowl shape, occurs in a CMP process due to a large size of the pattern, as shown in FIG. 8, and measurement accuracy becomes deteriorated in case of measuring the film thickness and the like by having an interface with the metal interconnection 302 as a reflective surface.
Therefore, in order to measure and control the thickness and the cross sectional shape of the film formed on the multilayer interconnection, it is required to develop a method for nondestructively measuring any position of a product wafer with high throughput.